(1) Field of the Invention
This invention relates to electrochemical gas detectors and, more particularly, to a three electrode electromechanical cell having a compact design.
(2) Background Art
The theory and general operation of the three electrode electrochemical cells used in toxic or other gas detector systems are well known. Such cells include a sensing electrode, a counter electrode spaced from the sensing electrode, an aqueous-based electrolyte between and in contact with the sensing and counter electrodes, and a reference electrode positioned between the sensing and counter electrodes and in contact with the electrolyte. The sensing electrode is typically a porous, gas diffusion electrode having a coating of a catalytic material on a surface adjacent the electrolyte. The gas to be sensed, referred to as the object gas, flows either alone or in combination with other gases (e.g., the ambient atmosphere) through the sensing electrode. The object gas undergoes a reaction, either a reduction or an oxidation, at the interface of the electrolyte and the catalytic material of the sensing electrode. The catalytic material is selected to promote reaction with a particular object gas, but not to react with other gases that may accompany the object gas.
The sensing electrode will function electrochemically as an anode if the reaction occurring at the interface of the electrolyte and catalytic material is an oxidation, for example, if the cell is used to detect carbon monoxide, hydrogen, hydrogen sulfide, hydrocarbons, or other fuel gases that are readily oxidizable. In an oxidation reaction, electrons are removed from the molecules of the object gas. The sensing electrode will function electrochemically as a cathode if the reaction occurring at the interface of the electrolyte and catalytic material is a reduction, for example, if the cell is used to detect nitrogen dioxide, chlorine, fluorine, bromine, oxygen or other oxidant gases that are readily reducible. In a reduction reaction, electrons are added to the molecules of the object gas.
The result of either an oxidation or a reduction reaction at the sensing electrode is the production of charged particles in the form of ions. Charged particles will migrate through the electrolyte to the counter electrode. A conductive wire is typically connected external of the cell between the sensing and counter electrodes to complete an electrical path, to allow electrons to flow between the counter and sensing electrodes and to permit an opposite electrochemical reaction to take place at the counter electrode.
The reference electrode is used to maintain a particular potential difference between the sensing and reference electrodes and to assist or encourage the oxidation or reduction reaction at the sensing electrode. For example, in a carbon monoxide (CO) detecting cell, the sensing electrode is fixed at a potential that makes it run in an electron hungry mode in which CO, the object gas, will readily be oxidized and give up electrons. The reference electrode is also used to bias the sensing electrode at a particular level so that other reactions will not take place and, thereby, will not interfere with the desired reaction resulting from the presence of the object gas. The reference electrode potential will be sensed without drawing significant current and will not affect the magnitude of the current generated by the reaction of the object gas at the sensing electrode.
With all other conditions remaining constant, such as temperature, gas pressure, and humidity, the electrons generated by the reactions within the cell will be directly proportional to the amount of object gas contacting the sensing electrode. The current flowing through the external circuit between the sensing and counter electrodes can be measured by an ammeter or the like and give a quantitative reading of the level of object gas present.
Prior art three electrode cells useful for detecting an object gas in an atmosphere are shown, for example, in U.S. Pats. Nos. Re. 31,914; Re. 31,915; and Re. 31,916. However, these cells are rather bulky due to the positioning of the electrodes with respect to each other and with respect to an electrolyte reservoir in the cells since the electrodes are all exposed to atmospheric air.
It is, accordingly, an object of the present invention to provide a three electrode electrochemical cell for gas detection which has a compact design.
The prior art has attempted to provide a three electrode electrochemical cell of compact design, but these designs all have various disadvantages or unsatisfactory characteristics. U.S. Pat. No. 4,521,290 discloses a thin layer electrochemical cell for detecting toxic chemicals. However, this cell includes a plurality of counter electrodes which are spaced apart from the remainder of the cell and are located in an electrolyte reservoir. Such an arrangement does not permit the construction of a compact cell. The cell disclosed in United Kingdom Patent Application No. 2,140,566A includes a working or sensing electrode spaced from flat reference and counter electrodes by a powdered, semisolid electrolyte. These electrodes are each positioned on the outer surface of the electrolyte. This cell includes no reservoir of electrolyte and does not adjust at all for variations in humidity.
City Technology Limited, of London, England, markets a compact CO cell under Model Nos. 3ER and 3FR, and markets a compact H.sub.2 S cell under Model No. 3HR. Similar cells are disclosed in U.S. Pats. Nos. 4,633,704, 4,587,003 and 4,406,770. These cells included porous, gas diffusion electrodes for the sensing, reference and counter electrodes and the electrodes are positioned in a stacked arrangement. However, these cells are not the most compact design available, utilize exclusively more expensive gas diffusion electrodes, weep liquid electrolyte after being used for a short time, and do not adequately compensate for variations in ambient humidity. In addition, the porous, hydrophobic reference and counter electrodes trap gas bubbles therein and, as a result, block off a portion of the catalytic material on the electrodes and interfere with the desired electrochemical reaction. Moreover, these cells provide a separate pathway between the counter electrode and the surrounding atmosphere.
It is a further object of the present invention to provide a compact, three electrode electrochemical cell which includes stacked electrodes, yet will operate efficiently, is inexpensive to manufacture, does not weep electrolyte, compensates readily for humidity changes and will operate in an ambient atmosphere having a high relative humidity.